Vacuum pump having expansion chamber and method of achieving ultimate pressure state in a vacuum pump using an expansion chamber

ABSTRACT

A vacuum pump and a method of controlling the operation of the vacuum pump employ an expansion chamber to reduce a pressure differential between an inlet side and an exhaust side of the pump. In particular, the expansion chamber may be used to rid the pump head of the pump of a pocket of relatively high pressure fluid that accumulates behind a poppet valve disposed over an exhaust opening. The pump itself may be used to create a level of vacuum pressure in the expansion chamber. Alternatively, a piston or diaphragm may be used. In either case, the expansion chamber is isolated from the ambient and interior of the pump head once a level of vacuum pressure is produced in the expansion chamber. Then the exhaust outlet is redirected from the ambient to the interior of the expansion chamber.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to vacuum pumps such as vacuum scroll pumps. In particular, the present invention relates to a method of operating a vacuum pump to control the pressure differential between inlet and exhaust sides of the pump.

2. Description of the Related Art

A scroll pump is a type of pump that includes a stationary plate scroll having a spiral stationary scroll blade, an orbiting plate scroll having a spiral orbiting scroll blade, and an eccentric driving mechanism to which the orbiting plate scroll is coupled. The stationary and orbiting scroll blades are nested with a radial clearance and predetermined relative angular positioning such that a pocket (or pockets) is delimited by and between the blades. The orbiting scroll plate and hence, the orbiting scroll blade, is driven by the eccentric driving mechanism to orbit about a longitudinal axis of the pump passing through the axial center of the stationary scroll blade. As a result, the volume of the pocket(s) delimited by the scroll blades of the pump is varied as the orbiting scroll blade moves relative to the stationary scroll blade. The orbiting motion of the orbiting scroll blade also causes the pocket(s) to move within the pump head assembly such that the pocket(s) is selectively placed in open communication with an inlet and outlet of the scroll pump.

In an example of such a scroll pump, the motion of the orbiting scroll blade relative to the stationary scroll blade causes a pocket sealed off from the outlet of the pump and in open communication with the inlet of the pump to expand. Accordingly, fluid is drawn into the pocket through the inlet. Then the pocket is moved to a position at which it is sealed off from the inlet of the pump and is in open communication with the outlet of the pump, and at the same time the pocket is contracted. Thus, the fluid in the pocket is compressed and thereby discharged through the outlet of the pump.

In the case of a vacuum-type of scroll pump, the inlet of the pump is connected to a system that is to be evacuated, e.g., a system including a processing chamber in which a vacuum is to be created and/or from which gas is to be discharged.

Like other types of vacuum pumps, vacuum scroll pumps have internal clearances that allow for a backflow of fluid in the pump in the case in which pressure at an exhaust side of the pump exceeds pressure at an inlet side of the pump by a certain amount.

SUMMARY OF THE INVENTION

It is a general object of the present invention to provide a vacuum pump that can consistently meet its ultimate pressure requirements.

Likewise, it is an object of the present invention to provide a method of controlling the operation of a vacuum pump so that the pump is operating optimally with respect to the state of pressure it is intended to produce.

Another object of the present invention is to minimize or prevent leakage from an exhaust side to an inlet side of a vacuum pump having internal clearances in its compression mechanism.

Still another object of the present invention is to provide means for and a method of decreasing a pressure differential between an inlet side and an exhaust side of a vacuum pump.

Yet another object of the present invention is to provide a vacuum scroll pump that can create a high level of vacuum pressure even with relatively large internal clearances between its orbiting and stationary plate scrolls.

Another object of the present invention is to provide a vacuum scroll pump that can create a high level of vacuum pressure with a minimal number of wraps of its scroll blades.

According to a general aspect of the present invention, each of these objects may be achieved without the need to operate a secondary pumping mechanism for a prolonged period of time.

According to one aspect of the present invention, there is provided a vacuum pump which includes an inlet portion having a pump inlet and constituting a vacuum side of the pump where fluid is drawn into the pump, an exhaust portion having a pump outlet and constituting a compression side where fluid is discharged under pressure from the pump, a pump head having an inlet opening to which the pump inlet extends, an exhaust opening leading to the pump outlet, and a compression mechanism constituted by at least one pocket whose volume is expanded while communicating with the inlet opening and reduced while communicating with the outlet opening, a poppet valve having a valve head seated over the exhaust opening of the pump head, an expansion chamber disposed outside the pump head and having chamber walls delimiting an interior space, vacuum pressure control means for selectively producing a vacuum in the expansion chamber, and fluid directional flow control means for selectively placing the pump in first and second states. The first state is one in which the exhaust opening of the pump head is in open communication with the interior space of the expansion chamber while the exhaust opening and the interior space of the expansion chamber are each closed to the pump outlet. The second state is one in which the exhaust opening of the pump head is in open communication with the pump outlet while the exhaust opening of the pump head is closed to the interior of the expansion chamber and the interior space of the expansion chamber is closed to the pump outlet.

According to another aspect of the invention, there is provided vacuum pump which includes an inlet portion having a pump inlet and constituting a vacuum side of the pump where fluid is drawn into the pump, an exhaust portion having a pump outlet and constituting a compression side where fluid is discharged under pressure from the pump, a pump head having an inlet opening to which the pump inlet extends, an exhaust opening leading to the pump outlet, and a compression mechanism constituted by at least one pocket whose volume is expanded while communicating with the inlet opening and reduced while communicating with the outlet opening, an expansion chamber disposed outside the pump head and having chamber walls delimiting an internal space of an unchangeable volume, and a system of passageways including a first passageway extending between the expansion chamber and the outlet portion of the pump head without passing through the compression mechanism of the pump constituted by said at least one pocket, and a second passageway by which the interior of the expansion chamber is connected to the compression mechanism of the pump head at a location upstream of the exhaust opening of the pump head.

According to still another aspect of the invention, there is provided a method of operating a vacuum pump, including by operating the pump to draw fluid into the pump from a system, monitoring a pressure of the fluid in the system while the pump is being operated, evacuating an expansion chamber disposed outside the pump head of the pump until a first level of vacuum pressure exists within the expansion chamber and then isolating the interior of the expansion chamber to fix the vacuum pressure at the first level, decreasing a pressure differential between the vacuum side and the compression side of the vacuum pump by placing the exhaust outlet of the pump head in open fluid communication with the interior of the expansion chamber while preventing the fluid from flowing out the pump outlet when the pressure of the fluid in the system has stabilized and with the vacuum pressure within the expansion chamber at the first level.

With respect to these aspects, the vacuum pump may be a scroll vacuum pump in which the pump head includes a stationary plate scroll including a stationary scroll blade, and an orbiting plate scroll including an orbiting scroll blade nested with the stationary scroll blade. The compression mechanism of the pump is constituted by at least one pocket defined between the nested scroll blades.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects, features and advantages of the present invention will be better understood from the detailed description of the preferred embodiments thereof that follows with reference to the accompanying drawings, in which:

FIG. 1 is a schematic longitudinal sectional view of a scroll pump to which the present invention can be applied;

FIG. 2A is a longitudinal sectional view of a pump head and pump motor of the pump of FIG. 1;

FIG. 2B is a sectional view of part of the pump head shown in FIG. 2A, illustrating tip seals between the stationary plate scroll and the orbiting plate scroll;

FIG. 3 is a flowchart of an example of a method of regulating the pressure state of a vacuum scroll pump according to the present invention;

FIG. 4A is a block diagram of equipment including an embodiment of a vacuum scroll pump according to the present invention with valves of the pump in one state during the course of the method shown in FIG. 3;

FIG. 4B is a schematic diagram of the system of FIG. 4A showing another state that the valves of the vacuum scroll pump assume during the course of the method shown in FIG. 3;

FIG. 4C is a schematic diagram of the system of FIG. 4A showing yet another state that the valves of the vacuum scroll pump assume during the course of the method shown in FIG. 3;

FIG. 4D is a schematic diagram of the system of FIG. 4A showing the state that the valves of the vacuum scroll pump assume during another point in time during the course of the method shown in FIG. 3;

FIG. 5 is a block diagram of equipment including another example of a vacuum scroll pump according to the present invention; and

FIG. 6 is a block diagram of equipment including still another example of a vacuum scroll pump according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Various embodiments and examples of embodiments of the inventive concept will be described more fully hereinafter with reference to the accompanying drawings. In the drawings, the sizes and relative sizes of elements may be exaggerated for clarity. Likewise, the shapes of elements may be exaggerated and/or simplified for clarity and ease of understanding. Also, like numerals and reference characters are used to designate like elements throughout the drawings.

Furthermore, spatially relative terms, such as “front” and “back” are used to describe an element's relationship to another element(s) as illustrated in the figures. Thus, the spatially relative terms may apply to orientations in use which differ from the orientation depicted in the figures. Obviously, though, all such spatially relative terms refer to the orientation shown in the drawings for ease of description and are not necessarily limiting as apparatus according to the invention can assume orientations different than those illustrated in the drawings when in use.

Other terminology used herein for the purpose of describing particular examples or embodiments of the inventive concept is to be taken in context. For example, the terms “comprises” or “comprising” when used in this specification indicates the presence of stated features or processes but does not preclude the presence of additional features or processes. The term “fixed” may be used to describe a direct connection of two parts to one another in such a way that the parts can not move relative to one another or a connection of the parts through the intermediary of one or more additional parts in such a way that the parts can not move relative to each other.

Referring now to FIG. 1, in general, a scroll pump 1 to which the present invention can be applied includes a housing 100, and a pump head 200 and a pump motor 300 disposed in the housing 100. Furthermore, the housing 100 may define an air inlet 100A and an air outlet 100B at opposite ends thereof, respectively. The pump also has an inlet portion having a pump inlet 140 and constituting a vacuum side of the pump where fluid is drawn into the pump, and an exhaust portion having a pump outlet 150 and constituting a compression side where fluid is discharged under pressure from the pump.

The pump may also include a cooling fan 400 for cooling components of the pump including the pump head 200 and/or motor 300. The pump motor 300 may be an electric motor or an air motor. Also, the pump head 200, pump motor 300, and cooling fan 400 may be juxtaposed with one another along a longitudinal axis L of the pump, i.e., in an axial direction of the pump.

Referring to FIGS. 1 and 2A, the pump head 200 may include a frame 210, a stationary plate scroll 220, an orbiting plate scroll 230, and an eccentric drive mechanism 240.

In this case, the frame 210 may be of one unitary piece or may comprise several integral parts that are fixed to one another.

The stationary plate scroll 220 is fixed to the frame 210. The stationary plate scroll 220 comprises a stationary scroll blade 221. The orbiting plate scroll 230 comprises an orbiting scroll blade 231. The stationary scroll blade 221 and the orbiting scroll blade 231 are nested with a clearance and predetermined relative angular positioning such that a pocket or pockets P is/are delimited by and between the stationary and orbiting scroll blades 221, 231. In this respect, portions of the scroll blades 221 and 231 need not contact each other to seal the pocket(s) P. Rather, minute radial clearances between portions of the scroll blades 221 and 231 create a seal sufficient for forming a satisfactory pocket(s) P. The pocket(s) P constitute a compression mechanism 260 of the pump as will be referred to in more detail later on.

In addition, as shown in FIG. 2B, the pump head 200 also has a tip seal 290 to create an axial seal between the scroll blade of one of the orbiting and stationary plate scrolls and the (floor or plate) of the other of the orbiting and stationary plate scrolls. More specifically, the tip seal 290 is a plastic member seated in a groove in and running the length of the tip of the scroll blade 221, 231 of one of the stationary and orbiting plate scrolls 220, 230 so as to be interposed between the tip of the scroll blade 221, 231 and the plate of the other of the stationary and orbiting plate scrolls 220, 230. FIG. 2C shows tip seals 290 associated with both of the scroll blades 221, 231, respectively.

The eccentric drive mechanism 240 may include a drive shaft 241 and bearings 246. In this example, the drive shaft 241 is a crank shaft having a main portion 242 coupled to the motor 300 so as to be rotated by the motor about the longitudinal axis L of the pump 100, and a crank 243 whose central longitudinal axis is offset in a radial direction from the longitudinal axis L. Also, in this example, the main portion 242 of the crank shaft is supported by the frame 210 via one or more sets of the bearings 246 so as to be rotatable relative to the frame 210. The orbiting plate scroll 230 is mounted to the crank 243 via another set or sets of the bearings 246. Thus, the orbiting plate scroll 230 is carried by crank 243 so as to orbit about the longitudinal axis of the pump when the main shaft 242 is rotated by the motor 300, and the orbiting plate scroll 230 is supported by the crank 243 so as to be rotatable about the central longitudinal axis of the crank 243.

During a normal operation of the pump, loads on the orbiting scroll blade 231 tend to cause the orbiting scroll plate 230 to rotate about the central longitudinal axis of the crank 243. Therefore, a metallic bellows 250 and/or other means such as an Oldham coupling may be provided for restraining the orbiting plate scroll 230 in such a way as to allow it to orbit about the longitudinal axis L of the pump while inhibiting its rotation about the central longitudinal axis of the crank 243. In this example, the bellows 250 is fixed at one end thereof to the orbiting plate scroll 230 and at a second end thereof to the frame 210. In this respect, the metallic bellows 250 is radially flexible enough to allow the first end thereof to follow along with the orbiting plate scroll 230 while the second end of the bellows remains fixed to the frame 210. On the other hand, the metallic bellows 250 has a torsional stiffness that prevents the first end 251 of the bellows from rotating significantly about the central longitudinal axis of the bellows, i.e., from rotating significantly in its circumferential direction while the bellows remains fixed to the frame 210.

Furthermore, the bellows 250 may also seal the bearings 246 and bearing surfaces from a space defined between the bellows 250 and the frame 210 in the radial direction and which space constitutes a chamber C, i.e., a vacuum chamber of the pump, through which fluid worked by the pump passes. Accordingly, lubricant employed by the bearings 246 and/or particulate matter generated by the bearings surfaces can be prevented from passing into the chamber C by the bellows 250.

The pump head 200 also has an inlet opening 270 at the inlet side of the pump and to which the pump inlet 140 extends, and an exhaust opening 280 at the exhaust side of the pump and leading to the pump outlet 150.

The orbiting motion of the orbiting scroll blade 221 relative to the stationary scroll blade 231 causes a pocket P sealed off from the outlet 150 of the pump and in open communication with the inlet 140 of the pump to expand. Accordingly, fluid is drawn into the pocket P through the pump inlet 140. Then the pocket P is moved to a position at which it is sealed off from the inlet 140 of the pump and is in open communication with the pump outlet 150, and at the same time the pocket P is contracted. Thus, the fluid in the pocket P is compressed and thereby discharged from the pump through the outlet 150. Accordingly, the fluid moves through the pump as shown schematically in FIG. 1 by the arrow-headed lines.

Vacuum scroll pumps rely on the aforementioned small internal clearances and numbers of turns (also referred to as “wraps”) of the spiral scroll blades to generate the compression required to meet the ultimate pressure requirements of the pumps. In a broad sense, the compression ratio of a vacuum scroll pump is a value that represents the ratio of the volume of the compression mechanism of the pump (constituted by the pocket(s) P) from its largest capacity to its smallest capacity. Technically speaking, though, those of skill in the art will recognize that the compression ratio of a scroll vacuum pump is the ratio between outlet and inlet pressures under a given operating condition. In this respect, the compression ratio corresponds to the rate of transport of fluid (gas) from the inlet side of the pump to the outlet side of the pump, less any internal leakage that occurs, and a factor of the amount of volume reduction of the pockets from their size when fluid is taken in, versus their size when the pockets reach the pump outlet. The “ultimate pressure” of a scroll vacuum pump, on the other hand, is defined by the size of those leakages. More specifically, when the pump inlet is closed and no gas enters there, the ultimate pressure is the inlet pressure at which the (intended) pumping flow of fluid from the inlet to the outlet is equal to the (unintended) leakage of fluid in the reverse direction from the outlet toward the inlet.

Especially in the case in which the scroll pump is operating while meeting its ultimate pressure requirements, the inlet side of the scroll pump is at a low pressure, and the exhaust side of the pump is at a relatively high pressure (approximately atmospheric). The pressure differential from exhaust side to the inlet side creates a potential for leakage of the fluid in the pump in a direction from the exhaust side to the inlet side through the internal clearances between the plate scrolls. Furthermore, this potential for leakage is increased as the seals between the plate scrolls begin to wear. In any case, such a backflow of the fluid may not only affect the performance of the pump but may, in turn, upset the operation of the system connected to the pump.

An example of a method of means for reducing the pressure at the exhaust side of the pump and hence, for reducing the pressure differential between the inlet side and the outlet side of the pump, according to the present invention, will now be described with reference to FIG. 3 and FIGS. 4A-4D.

Referring first to FIG. 4A, an embodiment of the vacuum pump 1 according to the present invention includes an expansion chamber 500 disposed outside the pump head 200. The expansion chamber 500 may be provided within the housing 100 (refer back to FIG. 1) or may be mounted on the outside of the housing 100 or otherwise integral with the housing 100 outside the space in which the pump head 200 and pump motor 300, etc. are housed. In this embodiment, the volume of the interior of the expansion chamber is a fixed volume in that it can not be changed.

The pump may also include a poppet valve PV provided in-line between the exhaust opening 280 of the pump head 200 and the pump outlet 150. In this example, the valve head of the poppet valve PV is seated over the exhaust opening 280 and is spring-loaded such that the exhaust opening 280 is uncovered when the pressure on the exhaust opening 280 side of the valve head exceeds the pressure on the pump outlet 150 side by a certain amount. Accordingly, a pocket of relatively high pressure fluid tends to accumulate at the exhaust opening 280 side of the poppet valve PV during operation. This pocket of relatively high pressure fluid can serve as a root cause of fluid leakage back through the internal clearances of the pump head 200 as was discussed above.

The vacuum pump 1 also includes vacuum pressure control means for selectively producing a vacuum in the expansion chamber 500, and fluid directional control means for selectively placing the pump in a first state in which the exhaust opening 280 of the pump head 200 is in open communication with the interior space of the expansion chamber 500 while the exhaust opening 280 and the interior space of the expansion chamber 500 are each closed to the pump outlet 150 and a second state in which the exhaust opening 280 of the pump head 200 is in open communication with the pump outlet 150 while the exhaust opening 280 of the pump head 200 is closed to the interior of the expansion chamber 500 and the interior space of the expansion chamber 500 is closed to the pump outlet 150.

The fluid directional control means comprises at least one passageway and at least one valve. In this example, the fluid directional control means comprises a first passageway P1 that directly connects the expansion chamber 500 and the exhaust opening 280 of the pump head 200, and a two-way valve V2W disposed in the first passageway P1 (in-line between the expansion chamber 500 and the exhaust opening 280 of the pump head 200) and connected to the pump outlet 150. The fluid directional control means may instead, however, comprise several valves and passageways configured to provide the same selective control of the direction in which the fluid flows after leaving the pump head 200 as will be described in more detail later on.

The vacuum pressure control means in this example comprises a passageway P2 (second passageway) that directly connects the expansion chamber 500 and the pump inlet 140, and a valve V disposed in the second passageway P2.

FIG. 4A also shows the inlet 140 of the vacuum scroll pump 1 connected to a system 2 comprising a chamber from which fluid is to be discharged by the vacuum scroll pump 1. Furthermore, the vacuum scroll pump 1 and/or system 2 may comprise a controller 1000 and at least one pressure sensor for sensing the pressure at a location within the vacuum scroll pump 1 and/or system 2. The controller 1000 is operatively connected to the valves V and V2W and is configured to control the valves based on the pressure monitored by the at least one pressure sensor. An example of this control, for regulating the pressure state in the pump will now be described in detail.

Referring to FIGS. 3 and 4A, the control operation starts and at this time, valves V and V2W are in positions that isolate the expansion chamber 500 from the pump head 200, and the exhaust opening 280 of the pump head is open to the pump outlet 150, i.e., the pump outlet 150 can be said to be open. In this state, the vacuum scroll pump is run and a pumpdown of the system 2 begins to draw fluid into the vacuum scroll pump 1 from the system 2 and thereby creating vacuum pressure in the system 2 (S10).

At this time, a pressure of fluid in the system 2 is monitored by the controller 1000 on the basis of the pressure sensed by the pressure sensor(s). This monitoring may take place anywhere upstream of the pump inlet 140. The controller 1000 then determines (S20) whether the level of the vacuum pressure in the system has stabilized, typically at a level (e.g., 200 mTorr) within a predetermined range indicative of a relatively moderate level of vacuum pressure being produced in the system 2. At this time, the pressure differential between the inlet and exhaust sides of the vacuum pump 1 may become great enough, for reasons described above, to cause the rate at which the pump is pumping the fluid to stall or slow.

Accordingly, valve V is opened (FIG. 4B) to connect the interior of the expansion chamber 500 to the inlet 140 of the pump which is at a relatively high vacuum pressure. As a result, the expansion chamber 500 is evacuated to produce a level of vacuum pressure in the expansion chamber 500 (S30). That is, this embodiment may be characterized in that the operation of the vacuum pump 1 itself is used create a level of vacuum pressure within the expansion chamber 500. Then, a determination is again made by the controller 1000 as to whether level of vacuum pressure in the system 2 has stabilized to a level within the predetermined range (S40).

Next, the expansion chamber 500 is isolated from the pump head 200 (S50) by closing the valve V (FIG. 4C). Accordingly, the level of vacuum pressure in the expansion chamber 500 is fixed.

Then, the two-way valve V2W is moved to the position shown in FIG. 4D to redirect the exhaust opening 280 of the pump head 200 from the room (via the pump outlet 150) to the expansion chamber 500 (S60). As a result, a pocket of relatively high pressure fluid behind the poppet valve PV, i.e., at the exhaust outlet 280 of the pump head 200, is drawn into the expansion chamber 500 which is at vacuum pressure, and the pressure differential between the inlet and outlet side of the pump will decrease. Hence, the pressure in the system 2 will drop dramatically.

The pressure will be monitored (S70) until it has stabilized to a level within a predetermined range (e.g., 10-20 mTorr). At that time, the two-way valve V2W is returned to the position shown in FIG. 4A to redirect the exhaust opening 280 from the expansion chamber 500 to the room (S80). This process may be repeated as required until the vacuum scroll pump 1 is operating under an ultimate or optimal pressure regimen.

FIG. 5 shows equipment including another example of a vacuum pump 1′ according to the present invention. This example is similar to that of FIG. 4A except that the second passageway P2 connects the expansion chamber 500 and the compression mechanism 260 of the pump head 200 at a location at the beginning of or approximately mid-way in the compression cycle being carried out by the compression mechanism 260. The same method of control shown in and described with reference to FIG. 3 may be used with this pump 1′.

FIG. 6 shows equipment including another example of a vacuum pump 1″ according to the present invention. In this example, like those of the previous examples, the fluid directional flow control means comprises a passageway P that directly connects the expansion chamber 500″ and the exhaust opening 280 of the pump head 200, and a valve V2W disposed in the first passageway. On the other hand, the fluid pressure control means comprises a mechanism 510 that expands a volume of space in the expansion chamber 500″ to create a level of vacuum pressure in the chamber 500″. The mechanism 510 may comprise a diaphragm or piston (shown) or the like and an electrical, mechanical or pneumatic actuator for moving the diaphragm or piston under the control of the controller 1000.

The operation of the vacuum pump 1″ is controlled in a way similar to that shown in and described with reference to FIG. 3. In this case, however, the mechanism 510 is operated at S30 to produce a level of vacuum pressure in the expansion chamber 500″, and the mechanism 510 is not operated at the time exhaust opening 280 of the pump head is redirected from the room to the expansion chamber 500″ at S60.

Finally, embodiments of the inventive concept and examples thereof have been described above in detail. The inventive concept may, however, be embodied in many different forms and should not be construed as being limited to the embodiments described above. For example, although the present invention has been described in detail with respect to vacuum scroll pumps, the present invention may be applied to other types of vacuum pumps that include a compression mechanism constituted by at least one pocket whose volume is varied to draw fluid into the pump and expel the fluid from the pump. Accordingly, the embodiments and examples of the invention were described so that this disclosure is thorough and complete, and fully conveys the inventive concept to those skilled in the art. Thus, the true spirit and scope of the inventive concept is not limited by the embodiment and examples described above but by the following claims. 

What is claimed is:
 1. A vacuum pump comprising: an inlet portion having a pump inlet and constituting a vacuum side of the pump where fluid is drawn into the pump, and an exhaust portion having a pump outlet and constituting a compression side where fluid is discharged under pressure from the pump; a pump head having an inlet opening to which the pump inlet extends, an exhaust opening leading to the pump outlet, and a compression mechanism constituted by at least one pocket; a poppet valve having a valve head seated over the exhaust opening of the pump head; an expansion chamber disposed outside the pump head, the expansion chamber having chamber walls delimiting an interior space; vacuum pressure control means for selectively producing a vacuum in the expansion chamber; and fluid directional flow control means for selectively placing the pump in a first state in which the exhaust opening of the pump head is in open communication with the interior space of the expansion chamber while the exhaust opening and the interior space of the expansion chamber are each closed to the pump outlet and a second state in which the exhaust opening of the pump head is in open communication with the pump outlet while the exhaust opening of the pump head is closed to the interior of the expansion chamber and the interior space of the expansion chamber is closed to the pump outlet.
 2. The scroll pump as claimed in claim 1, wherein the volume of the interior space of the expansion chamber is unchangeable.
 3. The scroll pump as claimed in claim 2, wherein the fluid directional flow control means comprises a first passageway that extends between the expansion chamber and the exhaust opening of the pump head without passing through the compression mechanism of the pump constituted by said at least one pocket, and a valve disposed in the first passageway, and the fluid pressure control means comprises a second passageway that extends between the expansion chamber and the pump inlet without passing through said compression mechanism of the pump, and a valve disposed in said second passageway.
 4. The scroll pump as claimed in claim 2, wherein the fluid directional flow control means comprises a first passageway that extends between the expansion chamber and the exhaust opening of the pump head without passing through the compression mechanism of the pump constituted by said at least one pocket, and a valve disposed in the first passageway, and the fluid pressure control means comprises a second passageway that extends between the expansion chamber and said compression mechanism of the pump, and a valve disposed in said second passageway.
 5. The scroll pump as claimed in claim 1, wherein the fluid directional flow control means comprises a passageway that extends between the expansion chamber and the exhaust opening of the pump head without passing through the compression mechanism of the pump constituted by said at least one pocket, and a valve disposed in the first passageway, and the fluid pressure control means comprises a mechanism that expands a volume of space in the expansion chamber.
 6. The scroll pump as claimed in claim 5, wherein the mechanism comprises a diaphragm or piston, and an actuator connected to the diaphragm or piston.
 7. A vacuum pump comprising: an inlet portion having a pump inlet and constituting a vacuum side of the pump where fluid is drawn into the pump, and an exhaust portion having a pump outlet and constituting a compression side where fluid is discharged under pressure from the pump; a pump head having an inlet opening to which the pump inlet extends, an exhaust opening leading to the pump outlet, and a compression mechanism constituted by at least one pocket; an expansion chamber disposed outside the pump head, the expansion chamber having chamber walls delimiting an internal space of an unchangeable volume; and a system of passageways including a first passageway extending between the expansion chamber and the outlet portion of the pump head without passing through the compression mechanism of the pump constituted by said at least one pocket, and a second passageway by which the interior of the expansion chamber is connected to the compression mechanism of the pump head, constituted by the at least one pocket, at a location upstream of the exhaust opening of the pump head.
 8. The scroll pump as claimed in claim 7, further comprising valves disposed in the system of passageways and operable to selectively open and close the interior of the expansion chamber to the exhaust opening of the pump head, to selectively open and close the exhaust outlet of the pump head to the pump outlet, and to selectively open and close the second passageway between the expansion chamber and the compression mechanism of the pump head constituted by the at least one pocket.
 9. The scroll pump as claimed in claim 7, further comprising a first valve disposed in the first passageway an operable to selectively place the pump in a first state in which the exhaust opening of the pump head is in open communication with the interior of the expansion chamber while the exhaust opening and the interior of the expansion chamber are each closed to the pump outlet and a second state in which the exhaust opening of the pump head is in open communication with the pump outlet while the exhaust opening of the pump head is closed to the interior of the expansion chamber and the interior of the expansion chamber is closed to the pump outlet.
 10. The scroll pump as claimed in claim 7, wherein the second passageway leads directly to the pump inlet.
 11. The scroll pump as claimed in claim 7, wherein the second passageway leads directly into the compression mechanism of the pump head constituted by the at least one pocket.
 12. The scroll pump as claimed in claim 7, further comprising a poppet valve having a valve head seated over the exhaust opening of the pump head.
 13. A method of operating a vacuum pump comprising an inlet portion having a vacuum side where a vacuum is created to draw fluid into the pump, an exhaust portion constituting a compression side where the fluid drawn into the pump is discharged under pressure fluid from the pump, a pump head having an inlet opening to which the pump inlet extends, an exhaust opening leading to the pump outlet, and a compression mechanism constituted by at least one pocket, the method comprising: operating the pump to draw fluid into the pump from a system; monitoring a pressure of the fluid in the system while the pump is being operated; evacuating an expansion chamber disposed outside the pump head until a first level of vacuum pressure exists within the expansion chamber, and then isolating the interior of the expansion chamber from the ambient outside the pump and from the interior of the pump head so as to maintain the first level of vacuum pressure in the expansion chamber; when the pressure of the fluid in the system has stabilized to be within a first range and with the pressure in the expansion chamber at said first level, decreasing the pressure differential between the vacuum side and the compression side of the pump by placing the exhaust outlet of the pump head in open fluid communication with the interior of the expansion chamber while preventing the fluid from flowing out the pump outlet.
 14. The method of claim 13, further comprising: subsequently allowing fluid to flow out of the pump outlet and cutting off fluid communication between the expansion chamber and the exhaust outlet of the pump head when the pressure of the fluid flowing the pump reaches a second level.
 15. The method of claim 13, wherein the compression mechanism of the scroll pump is used to evacuate the expansion chamber to create the level of vacuum pressure in the expansion chamber.
 16. The method of claim 15, wherein the evacuating of the expansion chamber comprises placing the expansion chamber in fluid communication with the compression mechanism of the scroll pump via the pump inlet.
 17. The method of claim 15, wherein the evacuating of the expansion chamber comprises placing the expansion chamber in open fluid communication with the compression mechanism of the scroll pump at a location between the inlet and exhaust openings of the pump head.
 18. The method of claim 15, further comprising: subsequently allowing fluid to flow out of the pump outlet and cutting off fluid communication between the expansion chamber and the exhaust outlet of the pump head when the pressure of the fluid flowing the pump reaches a second level.
 19. The method of claim 13, wherein the evacuating of the expansion chamber to create the level of vacuum pressure in the expansion chamber comprises expanding a volume of space within the expansion chamber without venting the space.
 20. The method of claim 15, further comprising: subsequently allowing fluid to flow out of the pump outlet and cutting off fluid communication between the expansion chamber and the exhaust outlet of the pump head when the pressure of the fluid flowing the pump reaches a second level. 